In a digital communication system, frequency selectivity and temporal fluctuation of a transmission path occur because of multipath fading caused when a transmission signal reflects on buildings or the like or of Doppler fluctuation caused by movement of a terminal. In such a multipath environment, a reception signal becomes to a signal interfering with a transmission symbol and a symbol that arrives after the elapse of a delay time.
In the transmission path having such frequency selectivity, a single carrier (SC) block transmission system has attracted attention in recent years to obtain a best reception characteristic (see, for example, Non-Patent Literature 1). In the SC block transmission system, it is possible to reduce peak power as compared with an OFDM (Orthogonal Frequency Division Multiplexing) transmission system (see, for example, Non-Patent Literature 2) that is directed to multicarrier (Multiple Carrier: MC) block transmission.
In a transmitter that performs SC block transmission, for example, a multipath fading countermeasure takes place by performing transmission explained below. First of all, after a PSK (Phase Shift Keying) signal or QAM (Quadrature Amplitude Modulation) signal that is a digital modulation signal is generated in a “Modulator”, the digital modulation signal is converted into a time domain signal by a precoder and an IDFT (Inverse Discrete Fourier Transform) processing unit. Thereafter, a CP (Cyclic Prefix) is inserted in a CP inserting unit as a multipath fading countermeasure. The CP inserting unit copies a specified number of samples behind the time domain signal and adds the samples to the beginning of a transmission signal. To suppress transmission peak power, in the transmitter that performs the SC transmission, in general, DFT (Discrete Fourier Transform) processing is performed in the precoder.
In Non-Patent Literatures 1 and 2, the transmission peak power is suppressed while the influence of the multipath fading is reduced. However, in the SC block transmission, because phases and amplitudes between
SC blocks are discontinuous, an out-of-band spectrum or out-of-band leakage is caused. The out-of-band spectrum is interference to an adjacent channel. Therefore, out-of-band spectrum suppression is necessary. A spectrum mask is determined in a general communication system, and so it is necessary to suppress the out-of-band spectrum to satisfy the spectrum mask.
Non-Patent Literature 3 proposes a technique of inserting symbols formed by fixed sequences into both ends of a block to suppress the out-of-band spectrum. A transmitter described in Non-Patent Literature 3 generates data symbols and fixed sequence symbols for each block and multiplexes the data symbols and the fixed sequence symbols in a time domain. The data symbol is, for example, a symbol based on a modulation system such as PSK or QAM, and changes at random. The transmitter converts the multiplexed signals into a signal in a frequency domain through DFT processing, performs interpolation processing, for example, oversampling in the frequency domain, and converts the signal into a signal in a time domain through IDFT processing. The number of inputs and outputs of a DFT unit is represented as ND, the number of inputs of an interpolation processing unit is represented as ND, the number of outputs of the interpolation processing unit is represented as LN, the number of inputs and outputs of an IDFT unit is represented as LN, and an oversampling rate of the oversampling that is interpolation processing is set to L times. In the transmitter, N-point IDFT processing is carried out at the time of L=1, wherein N≥ND holds. In the case of N−ND>0,a zero is inserted into an output of the DFT unit in the interpolation processing unit. As a zero inserting method, for example, a method as described in Non-Patent Literature 4 is used.
An output of the IDFT unit is referred to as “sample”. The fixed sequence symbol mentioned before is formed of M symbols, in all blocks of which the same sequence is inserted in the same position. Because the same sequence is generated in generation of the fixed sequence symbols, a saved fixed sequence symbol may be read out from a memory. For the oversampling processing, any processing may be used, but in general, zero insertion or the like is used.
As explained above, ND symbols are inputted to the DFT unit, the symbols being obtained by multiplexing a data symbol and a fixed sequence symbol for one block. Because the number of symbols of the fixed sequence symbols is M, the number of symbols of the data symbols is ND−M. In Non-Patent Literature 3, the M fixed sequence symbols are divided into halves. For arrangement of the fixed sequence symbols in a block, M/2 symbols in the latter half of the fixed sequence symbols are arranged in a head part of the block before the ND−M data symbols arranged in the center of the block, while M/2 symbols in the former half of the fixed sequence symbols are arranged in a tail part of the block behind the ND−M data symbols. The fixed sequence symbols can be represented as, for example, F−M/2, F−M/2+1, . . . , F−1, F0, F1, . . . , FM/2−2, and FM/2−1. When a plurality of blocks are generated in the transmitter, the M/2 symbols F0, F1, . . . , FM/2−2, and FM/2−1 in the latter half of the fixed sequence symbols placed in the head part of the block are continuous from the M/2 symbols F−M/2, F−M/2+1, . . . , and F−1 in the former half of the fixed sequence symbols placed in the tail part of the immediately preceding block. For example, when a m-th data symbol in a k-th block is represented as dk,m, arrangement of the data symbols and the fixed sequence symbols before input to the DFT unit can be represented as F0, . . . , FM/2−1, dk, 1, . . . , dk,ND−M, F−M/2, . . . , and F−1 (ND is written as ND in the subscript) in the order of position from the head of the block. Any sequence may be used for the fixed sequence symbols, and a Zadoff-Chu sequence, zero, or the like may be used for the same.
In this way, the block in which the fixed sequence symbols are arranged as described in Non-Patent Literature 3 is inputted to the DFT unit, whereby phases between blocks link in an output of the IDFT unit, making it possible to suppress the out-of-band spectrum. In the example explained above, the fixed sequence symbols are arranged so that the number of symbols in the former half part is equal to the number of symbols in the latter half part. However, the number of symbols may be made different between the former half part and the latter half part.
A principle that waveform continuity is maintained by insertion of the fixed sequence symbols explained above is explained. In the block, a cyclicity phenomenon is caused by a combination of DFT processing, interpolation processing, and IDFT processing. In the cyclicity phenomenon caused by the combination of the processing, a waveform of each symbol is turned back to the opposite side of the block in the tail end of the block. By fixing first and last symbols of each block using such a characteristic, it is possible to smoothly link phases between the blocks.